Distribution transformers (50-2500 kVA)

The characteristic data of distribution transformers are determined by the requirements of the network. The effective power determined must be multiplied by the power factor cosφ to give the rated power SrT. In distribution networks the value uk= 6% is preferred.

Selection of distribution transformers for supplying power to LV networks (photo credit: gelexcambodia.com)

Transformer losses are made up of no-load losses and short circuit losses. The no-load losses are caused by the continuous reversal of magnetization of the iron core and are practically constant and independent of loading. The short circuit losses are made up of the ohmic losses in the windings and losses due to leakage ﬁelds. They are proportional to the square of the loading.

Oil-ﬁlled transformers and cast resin dry-type transformers are preferred. The use of askarel transformers is forbidden!

In this technical article, the most important criteria for the selection of distribution transformers in the power range from 50 to 2500 kVA for supplying power to low voltage networks will be discussed.

The installation of transformers should be free of underground water and ﬂooding. The cooling must be protected against sunlight. Fire protection measures and environmental compatibility must also be ensured. Figure 1 illustrates a transformer with oil ﬁlling < 1000 liters. Here, an impermeable ﬂoor is suﬂicient.

The heat losses of transformers must be properly dissipated! This requires air intake and exhaust openings. The air intake must flow in underneath the transformer close to the ﬂoor and the exhaust must be led upwards.

The size of the exhaust opening is shown without grate in Figure 2 for a room heating of 15 K.

Figure 2 – Ventilation for the interior installation of a cast resin transformer

Figure 3 gives the permissible overload capacity of cast resin transformers in the power range from 400 to 2500 kVA. The determination of the loading capacity of oil-ﬁlled transformers can be made on the basis of IEC 60076-1.

The heating losses which arise while operating a transformer (Figure 4) must be dissipated. If it is not possible owing to the conditions for installation to utilize natural ventilation, it is necessary to install a fan. The overall temperature of the transformer must not exceed 40°C.

Figure 4 – Ventilation of the transformer room

The overall losses in a transformer room are given by:

Qloss = ∑ Ploss

Ploss = P0 + 1.2 × Pk75 × (SAF / SAN)2

The total losses are dissipated through:

Qv = Qloss1 + Qloss2 + Qloss3

The individually dissipated amounts of heat can be calculated from the following:

Natural air current:Qloss1 = 0.098 × A1.2 × √HΔυL3

Losses dissipated by forced air currents (Figure 3):

Qloss3 = VL × CPL × ρ

Losses dissipated through walls and the ceiling (Figure 4):

Qloss2 = 0.7 × AW × KW × ΔυW + AD × KD × ΔυD

The meanings of the symbols are:

Pv – Transformer power loss in kW

Qv – Dissipated losses in kW

QW,D – Dissipated losses through walls and ceiling in kW

AW,D – Area ofwalls and ceiling in m2

KW,D – Heat transfer coeﬂicient in kW/m2K

SAF – Power for the cooling type AF in kVA

SAN – Power for the cooling type AN in kVA

VL – Air ﬂow rate in m3/s, m3/h

Qv1 – Part dissipated in natural air current in kW

Qv2 – Part dissipated through walls and ceiling in kW

Qv3 – Part dissipated in forced air current in kW

The transformer noise is a combination of magnetic noise and the noise of additional ventilation.

Figure 5 shows the noise level of different transformers according to IEC Publication 551. The magnetic noise is the result of oscillations of the iron core (induction-dependent) and depends on the material properties of the core laminations.

Figure 5 – Sound pressure level of transformers

The acoustical power (Figure 6) is a measure of the noise level produced by an acoustical source.

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Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on Google+